1. Technical Field
The present invention relates to an image processing controller which converts multi-gradation raster data to high-resolution binary image data and a printing apparatus provided with the image processing controller.
2. Related Art
A printing system is known in which a photoreceptor is exposed to light beams in a pattern according to print image data, thereby forming a print image. In this printing system, both a laser print head and a light-emitting-diode (LED) print head are used for irradiating the photoreceptor with light beams. The laser print head scans laser light in a main scanning direction thereby to form a raster line. On the other hand, the LED print head includes LED arrays arranged in the main scanning direction and individually emitting light thereby to form a raster line. The above-described difference in the hardware configuration between the laser and LED print heads results in the difference between print image data formats that can be output to both heads respectively. Accordingly, in order that print image data output to the laser print head may be used in the LED print head, the data format of the print image data needs to be converted. More specifically, print image data generated for the laser print head is multi-gradation low-resolution data. The multi-gradation low-resolution print image data for the laser print head needs to be converted to high-resolution binary data in order to be usable for the LED print head. When the aforesaid conversion is executable, hardware and software resources used in a printer using a laser print head can also be used by a printer provided with an LED print head. Japanese patent application publication JP-A-2003-32480 proposes an ordinary technique for converting multi-gradation low-resolution print image data to high-resolution binary print image data. In the disclosed technique, a shaggy detecting pattern or shaggy correcting pattern is used for two-dimensional image data or a pixel value of noted pixel is determined based on upper, lower, right and left adjacent pixels.
However, in the stage of output of print image data to the laser print head, the print image data is already decomposed into one-dimensional raster data having only information about main scanning direction location. As a result, the technique disclosed by the above-mentioned publication cannot be employed. More specifically, since raster data involves only location information in the main scanning direction, pixels can be arrayed in either vertical or main scanning direction but have difficulty in being arrayed in the other direction. Furthermore, printed result has variations in color strength or moiré.